Generation of multiple wavelengths in the mid infra-red (MIR) from a quantum cascade lasers has been realized by designing one single core capable of emitting two wavelengths simultaneously (see “Bidirectional semiconductor laser”, C. Gmachl et al, Nature 387, 777 (1999): one core designed such that opposite bias leads to emission of two different wavelengths), or by adding gratings with different periods to one single core with broad enough gain bandwidth. See, e.g., “High-performance, homogeneous broad-gain QCLs based on dual-upper-state design”, K. Fujita et al, Appl Phys Lett 96, 241107 (2010) and further work by the same group in Appl Phys Lett 98, 231102 (2011): one core which has a wide gain bandwidth and can be tuned to emit over a wide range of wavelengths (tuning range 330 cm-1 in the first work, 600 cm-1 in the second). See also “Broadband Distributed-Feedback Quantum Cascade Laser Array Operating From 8.0 to 9.8 μm”, B. G. Lee et al: one broad-gain core, array of parallel devices whose exact wavelength is controlled by a grating. In these cases, the wavelength coverage is only about 15-20% of the center wavelength.
Another technique consists in stacking different cores on top of one another, each emitting at a different wavelength. See C. Gmachl, D. L. Sivco, R. Colombelli, F. Capasso and A. Y. Cho, “Ultra-broadband semiconductor laser,” Nature, 415, 883-887, (2002) and C. Gmachl, D. L. Sivco, J. N., Baillargeon, A. L. Hutchinson, F. Capasso and A. Y. Cho, “Quantum cascade lasers with a heterogeneous cascade: two-wavelength operation,” Appl. Phys. Lett., vol. 79, 572-574, 2001. Here again, gratings of different periods can be added above the core to select precise wavelengths, this time within a somewhat wider range due to the different cores (30-40% of the center wavelength).